The present invention relates to a method of forming a homogeneous ceramic coating on a substrate. The method comprises depositing a preceramic coating on a substrate and then heating the substrate while directing a stream of cooling gas at the exterior surface of the preceramic coating such that a temperature gradient is developed in the coating. This temperature gradient allows the preceramic material near the substrate/coating interface to be converted to its ceramic form while deterring said conversion in the preceramic material near the exterior surface of the coating. The temperature gradient is then decreased over time such that all of the preceramic material ceramifies from the substrate outward to form a homogeneous coating on the substrate.
Numerous methods of depositing thin ceramic coatings on various substrates are known in the art. One such method involves dissolving a preceramic material in a solvent, applying the solution to a substrate, allowing the solvent to evaporate to deposit a preceramic coating on the substrate and then heating the coated substrate to a temperature sufficient to convert the preceramic coating to a ceramic coating. Such process are described, for example, in U.S. Pat. Nos. 4,749,631 and 4,756,977, both granted to Haluska et al. and assigned to Dow Corning Corporation, wherein the preceramic materials were silicate esters and hydrogen silsesquioxane resin, respectively.
Pyrolysis of preceramic materials in this type of process is generally performed by heating the coated substrate in a furnace under various gaseous environments to convert the material to a ceramic. When the coated substrate is processed in this manner, however, the coating begins to ceramify on the exterior surface resulting in the formation of a thin "skin" of dense ceramic material over the exterior surface of the coating. This skin prevents the escape of any volatile compounds which may be present or may be formed during pyrolysis and it inhibits diffusion of the gaseous environment into the coating. These trapped volatile compounds and the lack of gas diffusion, in turn, cause inhomogeneities in the resultant ceramic coating.
U.S. Pat. No. 5,059,448 describes a rapid thermal processing (RTP) technique for converting hydrogen silsesquioxane resin coatings to ceramic silica coatings. This technique decreases the thermal budget of a substrate by using high intensity radiation to rapidly heat (50.degree.-300.degree. C./sec) thin preceramic coatings to an elevated temperature for a time which allows the desired physical or chemical processes to be completed but not allow the substrate to be adversely affected. It is suggested therein that this technique may result in the coating ceramifying from the substrate outward.
The present inventor has now discovered that by using the method of this invention, ceramification of coatings can be controlled more effectively so that ceramification occurs sequentially from the substrate outward resulting in the formation of a high quality uniform coating.